New integration tests for public attestations.

[bddap]

A few more tests I think belong here:

• ensure attestations can be deleted by setting IrI on-chain to None
• for any Attestation a, deserialize(serialize(a)) == a
  • test the above where priority > 2^8-1 (want to make sure compact encoding is handled properly)
• create did, create attestation, assert exact contents of did document

Originally posted by @bddap in https://github.com/docknetwork/sdk/pull/233#discussion_r583074625